Inside the Randonneur, is a U shaped stiffener.
The stiffener is held in place with a stiffener sleeve and the stiffener can be removed from the bag.
ABS is the standard stiffener that I use and unless you request the polycarbonate, your bag will ship with an ABS stiffener.
It's black, shock resistant, and stiff.
Polycarbonate is lighter weight, clear, and less stiff.
If you have a FS (full size 6x10) randonneur rack, demi-porteur, or porteur rack, the polycarbonate will work fine.
Otherwise, use ABS for the extra support.
After several years of transporting watermelons, it is possible that the stiffener will crack or break. When/if it does, contact me and I'll make a new one for you, free of charge.
There is no additional charge for ABS or Polycarbonate stiffeners.